D. Litvinov

Karlsruhe Institute of Technology, Carlsruhe, Baden-Württemberg, Germany

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Publications (100)155.48 Total impact

  • Journal of Crystal Growth 02/2012; · 1.55 Impact Factor
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    ABSTRACT: We report on localization dynamics of excitons in ensembles of self–organized CdSe islands embedded in ZnSe. The experimental methods employed are temperature–dependent, spatially resolved photoluminescence (μ–PL), spatially integrated PL (macro-PL), as well as time–resolved PL (TRPL). We observe the well known non–monotonous shift of the PL maximum with temperature caused by redistribution of the excitons amongst the islands. The measured shift is compared with the exact shift of the bandgap deduced from μ–PL measurements and found to depend strongly on island size and distribution. These transport processes are recovered in the temporal evolution of the PL. The decaytime of the spectrally integrated PL reaches its maximum at exactly the same temperature at which the redhift of the macro–PL turns into a blueshift. In TRPL the PL–spectrum consists of two contributions. We put the emission on the high energy side down to excited states in the islands.
    International Journal of Modern Physics B 01/2012; 15(28n30). · 0.46 Impact Factor
  • Journal of Crystal Growth 01/2012; 338(1):283. · 1.55 Impact Factor
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    ABSTRACT: For a single In0.3Ga0.7As quantum dot in a spin light-emitting diode, we compare the nuclear spin polarization created by a spin polarized electrical current with the nuclear spin polarization originating from optically generated spin polarized carriers. As detection method we employ high-resolution optical spectroscopy of the Overhauser shift. We find that optically and electrically generated electron spin populations in the quantum dot result in a nuclear spin polarization of comparable magnitude, provided the injected electrons have the same spin polarization degree in both excitation modes. An asymmetric dependence of nuclear spin polarization on electron spin polarization is observed, consistent with the theoretical treatment. The results imply that nuclear spin polarization degrees of ∼58% can be achieved by purely electrical means in self-assembled quantum dots.
    Physical review. B, Condensed matter 09/2011; 84(12). · 3.77 Impact Factor
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    ABSTRACT: We observe ultraviolet photoluminescence from sputtered ZnO quantum dots which are fabricated with no annealing steps. The nanocrystals are embedded in amorphous SiO2 and exhibit a narrow size distribution of 3.5 ± 0.6 nm. Photoluminescence and transmittance measurements show a shift of ultraviolet emission and absorption of the dots compared to bulk ZnO material. This work paves the way for cheap nanooptical devices in the ultraviolet which are fabricated in a single sputtering run.
    Optics Express 01/2011; 19(2):1641-7. · 3.55 Impact Factor
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    ABSTRACT: In this study, we investigated pre-structured (100) GaAs sample surfaces with respect to subsequent site-selective quantum dot growth. Defects occurring in the GaAs buffer layer grown after pre-structuring are attributed to insufficient cleaning of the samples prior to regrowth. Successive cleaning steps were analyzed and optimized. A UV-ozone cleaning is performed at the end of sample preparation in order to get rid of remaining organic contamination.
    Nanoscale Research Letters 01/2011; 6(1):211. · 2.52 Impact Factor
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    ABSTRACT: In this study, we investigated the effect of in situ annealing on InAs quantum dots site-selectively grown on pre-structured GaAs substrates. A morphological transition is observed with original double dots merging into one single dot during annealing. This is accompanied by a reduction of quantum dots originally nucleating between defined sites. The photoluminescence intensity of annealed site-selective quantum dots is compared to annealed self-assembled dots with linewidths of single dot emission of about 170 and , respectively. UV-ozone cleaning is used to optimize the sample cleaning prior to quantum dot growth.
    Journal of Crystal Growth 01/2011; 323(1):187-190. · 1.55 Impact Factor
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    Journal of Crystal Growth 01/2011; · 1.55 Impact Factor
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    ABSTRACT: Transmission electron microscopy (TEM) was applied to study In segregation during molecular beam epitaxy (MBE) growth of InAs-based quantum-dot (QD) structures with a focus on achieving QD luminescence at the technologically relevant wavelength of 1.3 μm. Quantitative composition analyses were carried out with the composition evaluation by lattice fringe analysis (CELFA) technique and In-segregation efficiencies were determined on the basis of atomic-scale composition profiles. The effect of the InAs-deposition rate on the In distribution was studied in detail which needs to be low, in the 0.005 ML/s regime to achieve long-wavelength emission. Further minimization of In-segregation induced intermixing can be achieved by lowering the substrate temperature during GaAs cap layer growth. Finally, a procedure is presented to extract the real composition of QDs taking into account their three-dimensional morphology and embedding in a GaAs cap layer.
    Journal of Physics Conference Series 02/2010; 209(1):012006.
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    ABSTRACT: We present time-resolved photoluminescence measurements performed on an ensemble of InAs quantum dots with density of 1011 dots/cm2 and ground state transition energies centered at 1.216 eV. The wavelength of the 100fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon assisted) photoluminescence (PL). The PL was detected with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal's cleave axes). The decay of the PL was time-resolved with a streak camera in the interval 1.5 - 3ns to avoid scattered laser light. A strong polarization dependence was observed. Considerable amount of the resonant fluorescence signal and even of the non-resonant PL signatures remained linearly polarized on a nanosecond time scale. A phenomenological rate equation analysis is made.
    Proc. SPIE 7597; 01/2010
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    ABSTRACT: In an effort to understand why short wavelength (∼1000 nm) GaAs-based photonic crystal slab nanocavities have much lower quality factors (Q) than predicted (and observed in Si), many samples were grown, fabricated into nanocavities, and studied by atomic force, transmission electron, and scanning electron microscopy as well as optical spectroscopy. The top surface of the AlGaAs sacrificial layer can be rough even when the top of the slab is smooth; growth conditions are reported that reduce the AlGaAs roughness by an order of magnitude, but this had little effect on Q. The removal of the sacrificial layer by hydrogen fluoride can leave behind a residue; potassium hydroxide completely removes the residue, resulting in higher Qs.
    Photonics and Nanostructures - Fundamentals and Applications 01/2010; · 1.79 Impact Factor
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    ABSTRACT: The In-concentration in InGaAs quantum dots located within a GaAs matrix was determined with the composition evaluation by lattice fringe analysis (CELFA) technique. However, the results obtained with this method cannot account for the three-dimensional shape of quantum dots and their embedding in GaAs. A correction procedure was developed that takes into consideration the shape of the quantum dots and the TEM sample thickness and quantum-dot size. After correction, In-concentration profiles show an increasing In-content towards the top of the quantum dots which is consistent with the effect of In-segregation and earlier studies using other experimental techniques. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Crystal Research and Technology 09/2009; 44(10):1083 - 1088. · 1.12 Impact Factor
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    ABSTRACT: An ensemble of InAs quantum dots with ground state transition energies centered at 1.216 eV and density 1011dots/cm2 has been studied by time-resolved photoluminescence (PL). The wavelength of the 100-fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon sideband) PL. The decay of the PL was time resolved with a streak camera in the interval 1.5–3 ns to avoid scattered laser light. The intensity of the PL was recorded with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal’s cleave axes); the ratio is 2.22 at low temperatures and low excitation. A phenomenological rate equation analysis is made, separating the excitations into two classes, one polarized along the excitation polarization and the other unpolarized (either that way immediately after the excitation pulse or scattered from the first class). Excellent fits to the data lead to the conclusion that both classes decay radiatively with a lifetime of 1 ns, and a transfer from the polarized to the unpolarized species takes place with a distribution time of 12 ns at low temperatures and low excitation, dropping rapidly toward zero for temperatures above 30 K and for intense excitation levels. The polarization of a coherently excited ground state exciton should dephase with a rate equal to the sum of the radiative rate plus the inverse of this distribution time.
    Solid State Communications 09/2009; 149:1485-1492. · 1.53 Impact Factor
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    ABSTRACT: An instability in the growth of nonperiodic InGaAs/GaAs multiple quantum well samples, ordinarily of high-quality when grown with equal periods of order of half the wavelength of light in the material, leads to a dramatic microscopic, self-organized surface grating. This effect was discovered while growing quantum wells with two unequal barrier lengths arranged in a Fibonacci sequence to form an optical quasicrystal. A laser beam incident normal to the surface of the sample is diffracted into a propeller-shaped pattern. The sample surface has a distinctly cloudy appearance when viewed along one crystal axis but is mirror-like when the sample is rotated 90 degrees. The instability results in a five-fold increase in the absorption linewidth of the heavy-hole exciton transition. Atomic force microscopy, transmission electron microscopy, and scanning electron microscopy were used to study the samples.
    Optics Express 01/2009; 16(26):21512-21. · 3.55 Impact Factor
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    ABSTRACT: In the context of a potential future quantum information processing we investigate the concurrent initialization of electronic spin states in InGaAs quantum dots (QDs) via electrical injection∈dex{spin!injection} from ZnMn(S)Se spin aligners. Single dots can be read out optically through metallic apertures on top of our spin-injection light-emitting diodes (spin-LEDs). A reproducible spin polarization degree close to 100% is observed for a subset of the QD ensemble. However, the average polarization degree is lower and drops with increasing QD emission wavelength. Our measurements suggest that ∈dex{spin!relaxation}spin relaxation processes outside the QDs, related to the energetic position of the electron quasi-Fermi level, as well as defect-related spin scattering at the III–V/II–VI interface should be responsible for this effect, leading us to an improved device design. Finally, we present first time-resolved ∈dex{electroluminescence measurement}electroluminescence measurements of the polarization dynamics using ns-pulsed electrical excitation. The latter should not only enable us to gain a more detailed understanding of the spin and carrier relaxation processes in our devices. They are also the first step towards future time-resolved optical and electrical spin manipulation experiments.
    11/2008: pages 103-114;
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    ABSTRACT: InAs quantum dot (QD) layers grown by molecular-beam epitaxy were investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. To achieve the highest possible In concentration in the QDs, InGaAs (instead of GaAs) cap layers with different In concentrations were deposited after the growth of the InAs QD layer. We combine different TEM techniques to determine the shape, size, and composition of the QDs. By applying a post-processing procedure, we are able to reconstruct the In concentration in the QDs which is measured too low in TEM due to the embedding of the QDs in material with lower In concentration and averaging along the finite TEM sample thickness. The determination of the composition of the layers on an atomic scale shows that the In concentration in the QDs increases in growth direction and reaches values up to 90%. Redistribution of indium during the InGaAs cap layer growth leads to a decrease of the In concentration in the cap layer with respect to the nominal In concentration. The observed redshift of the PL peak with increasing In concentration in the cap layer is attributed to the enlargement of island size and the change of the strain in the QD layers.
    Journal of Applied Physics 04/2008; 103(8):083532-083532-8. · 2.21 Impact Factor
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    ABSTRACT: We have studied the crystal structure of GaAs nanowhiskers grown by molecular beam epitaxy (MBE) on gold-activated GaAs(111)B substrates. The results of reflection high-energy electron diffraction and transmission electron microscopy showed that the MBE-grown GaAs nanowhiskers can form a crystal structure of sphalerite, wurtzite, or an intermediate phase close to 4H polytype, depending on the deposition conditions and the size of catalyst droplets. The results are interpreted within the framework of a thermodynamic model.
    Technical Physics Letters 01/2008; 34(6):538-541. · 0.56 Impact Factor
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    ABSTRACT: We report on the investigation of CdSe/ZnSe heterostructures by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). CdSe layers with nominal thicknesses tCdSe between 0.5 and 8 monolayers (ML) were embedded in a ZnSe matrix and grown on a GaAs(001) substrate by molecular-beam epitaxy at 280 °C. The Cd-distribution was obtained from high-resolution TEM lattice fringe images using composition evaluation by lattice fringe analysis technique. The measured minimum, average and maximum Cd-concentrations and the overall CdSe contents in the layers increase with the nominal CdSe layer thickness and reach a constant value at tCdSe = 4 ML. The measured CdSe content in the regions with the maximal Cd-concentration continues to increase for tCdSe ≥ 5 ML. The increasing of measured Cd-concentrations/CdSe contents is correlated with a red shift of PL spectra. In the CdSe layers with tCdSe ≥ 5 ML, formation of defects is observed. The increasing density of defects and decreasing intensity of PL spectra with tCdSe suggest that the critical thickness for defect formation during CdSe growth on the ZnSe(001) is between 4 ML and 5 ML. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (a) 01/2008; 205(12):2892-2897. · 1.21 Impact Factor
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    ABSTRACT: We present a detailed transmission electron microscopy study of the structure and distribution of self-organized Fe-rich nanocrystals in (Ga,Fe)N grown via metal-organic vapor phase epitaxy (MOVPE). In samples with a concentration of magnetic ions above the solubility limit-x >= 0.4%-of Fe in GaN at the growth conditions we can identify three different phases of the Fe-rich enclosures, namely: (a) alpha-Fe segregated in proximity of the surface, (b) epsilon-Fe3N in the volume beneath, though well above the nominal interface between the undoped GaN buffer and the Fe-doped overlayer, and (c) nanocrystals at an intermediate location showing the presence of adjacent alpha-Fe and epsilon-Fe3N. These observations suggest that nitrogen evaporation at the growing surface play an important role in the formation of Fe-rich phases. The occurrence of alpha-Fe and epsilon-Fe3N in the same nanocrystal indicates that the two phases are actually correlated: phase separation may first lead to the precipitation of epsilon-Fe3N which inherits the hexagonal structure of GaN but generates misfit dislocations, and then a further depletion of nitrogen promotes the conversion of epsilon-Fe3N into alpha-Fe. (C) 2008 Elsevier B.V. All rights reserved.
    JOURNAL OF CRYSTAL GROWTH. 01/2008; 310(14):3294-3298.
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    ABSTRACT: The influence of the conditions during growth of InAs/GaAs quantum-dot structures on GaAs(001) by molecular-beam epitaxy was investigated systematically with respect to achieving quantum-dot photoluminescence in the 1 eV range. The growth temperature, As flux, growth rate, InAs deposit, and growth interruption time before cap layer growth were varied. Photoluminescence spectroscopy and transmission electron microscopy were used to study the optical and structural properties. Large InAs quantum dots with photoluminescence in the 1 eV range were obtained at a low growth rate of 0.0056 ML/s. Analyzing in particular the low-growth-rate regime, we found that an InAs deposition of at least 2.4 ML and a growth temperature of 500−510 °C were crucial to obtain large quantum dots with a high size uniformity. Composition analyses by transmission electron microscopy revealed a significantly higher In concentration in the quantum dots grown at low growth rate compared to high-growth-rate samples.
    Journal of Applied Physics 10/2007; 102(7):073511-073511-9. · 2.21 Impact Factor

Publication Stats

608 Citations
155.48 Total Impact Points

Institutions

  • 1999–2012
    • Karlsruhe Institute of Technology
      • • Laboratory for Electron Microscopy
      • • Institute of Applied Physics
      Carlsruhe, Baden-Württemberg, Germany
  • 2006
    • Russian Academy of Sciences
      • Ioffe Physical-Technical Institute
      Moskva, Moscow, Russia
  • 2005
    • Technische Universität Dortmund
      Dortmund, North Rhine-Westphalia, Germany
  • 2000
    • Universität Paderborn
      • Department of Physics
      Paderborn, North Rhine-Westphalia, Germany
    • Technische Universität Berlin
      • Department of solid state Physics
      Berlin, Land Berlin, Germany
    • Tohoku University
      • Institute for Materials Research
      Sendai, Kagoshima-ken, Japan